Field of the Invention
The present invention relates to an electron microscope and measurement method.
Description of Related Art
In scanning transmission electron microscopy (STEM), the differential phase contrast (DPC) method is known as a technique for visualizing electromagnetic fields. In this technique, an amount of deflection occurring when an electron beam is transmitted through a sample is measured, and the electromagnetic field present within the sample that causes the deflection of the beam is calculated (see, for example, JP-A-2009-277618).
FIG. 13 schematically illustrates the operation of a conventional electron microscope, 101, performed when measurements are made by the DPC method using a segmented detector 104. Note that FIG. 13 shows only main portions of the electron microscope 101. FIG. 14 is a schematic representation of the segmented detector 104.
As shown in FIG. 13, an electron beam EB produced by an electron beam source (not shown) is focused onto a sample S by an illumination lens system (not shown). At this time, the angle of convergence is limited by a condenser aperture 102. A part of the electron beam EB transmitted through the sample S is detected by the segmented detector 104 located behind the sample S.
As shown in FIG. 14, the segmented detector 104 has four detector segments D1, D2, D3, and D4 obtained by circumferentially and equally dividing an annular detection surface 103 into four. The amounts of electrons impinging on the detector segments D1, D2, D3, and D4, respectively, can be detected at the same time. The optical distance (camera length) between the sample S and the segmented detector 104 can be adjusted with an imaging lens system (not shown).
Where a measurement is made using a DPC method, the camera length is so adjusted that the periphery of the cross section of the transmitted electron beam lies within the detection surface 103 of the segmented detector 104 as shown in FIG. 14. When the electron beam EB is deflected by the sample S, the position of the transmitted electron beam on the detection surface 103 deviates, whereby detection signals arising from the four separate detector segments D1-D4 increase or decrease. In the example shown in FIG. 14, an area E1 on the detection surface 103 irradiated with the transmitted electron beam is deflected by the sample S to an area E2.
It is possible to know the amount of deflection of the electron beam EB by extracting increases and decreases in the detection signals arising from the detector segments D1-D4 through additions and subtractions and thus the distribution of the electromagnetic field within the sample S can be found. Because the periphery of the cross section of the transmitted electron beam lies within the detection surface 103, the increases and decreases in the detection signals from the detector segments D1-D4 produced when the irradiated area moves can be increased.
Where the amount of deflection of the electron beam EB caused by the sample S is small, the amount of movement of the transmitted electron beam on the detection surface 103 is also small and, therefore, it is more difficult to detect the amount of movement. In this case, as shown in FIG. 15, it is effective to increase the amount of motion of the electron beam on the detection surface 103 by increasing the camera length.
However, if the camera length is increased without varying the illumination system, transmitted electrons are distributed over a wider area, i.e., the diameter of the electron beam increases. Therefore, as shown in FIG. 15, the transmitted electron beam illuminates the whole detection surface 103. This reduces the ratios of the amounts of variation of signals from the detector segments D1-D4 to the amount of motion of the irradiated area on the detection surface 103. Consequently, it is more difficult to measure the amount of deflection of the electron beam EB with high sensitivity.
Accordingly, it is conceivable to adopt a technique of reducing the angle of impingement on the sample S, for example, by reducing the hole diameter in the aperture of the illumination system as shown in FIG. 16. This can enhance the sensitivity with which the amount of deflection of the electron beam EB caused by the sample S is detected. Nonetheless, the angle of convergence decreases and so diffraction deteriorates the positional resolution.
In this way, with the conventional electron microscope, if the camera length is increased in an attempt to enhance the sensitivity with which the amount of deflection of the electron beam made by the electromagnetic field within the sample is detected, the angle of convergence must be reduced according to the size of the detection surface 103, thus restricting the positional resolution.